Logs model date and wall clock time to stdout using input argument
year, month, day and sec and also output the average and
instantaneous time difference between calls to this routine. Include
str in output.

Generally it is expected that this routine is called periodically, eg. once per day, in
which the average and instantaneous time difference info becomes quite
useful.

If optional argument tic_ext is present, use it to determine when
last call was made. If using tic_ext, an initial count
value of less than zero implies this is the first call, thus accum and n
are set zero.